This research proposes to examine the role that catheptic proteases play in the turnover of cellular enzymes (e.g. PEP Carboxylase) during the synchronous cell cycle of the eukaryote Chlamydomonas reinhardtii. The specific objectives will be to establish isoenzyme B of PEP carboxylase as a model substrate for the study of enzyme turnover during growth and development. The function of multiple forms of cathepsin D in this process will be studied. It is hoped also to isolate lysosomal vessicles from different stages of development in order to study in vitro degradation of natural protein substrates and the transport of such substrates into the lysosomal vessicles.